The invention relates to hydraulic holding down devices intended to counteract the lift off of fuel assemblies under the action of the forces exerted thereon by the coolant flowing upwards in the core. These devices are designed for causing the appearance of a hydraulic force directed downwards and exerted on the assemblies, opposing that due to the pressure drop undergone by the coolant when passing through the core or at least a part thereof. It finds a particularly important application. cation in light water cooled and moderated reactors whose assemblies are without a housing.
It relates more particularly to a device for hydraulically holding down a fuel assembly in a nuclear reactor comprising a lower core support plate, fuel assemblies without housing each comprising a bundle of elements held in position by a frame having a foot bearing on the plate and means for bringing the coolant below the plate and collecting the coolant which has flowed through the core in a coolant collector situated above the core, the foot of each assembly comprising a central coolant intake passage connected to the space below the plate, having a section less than the current section of the assembly and surrounded by a space limited by the lower face of an annular zone of the foot connected to the collector.
U.S. Pat. No. 4,174,257 describes a device of this kind which comprises a simple lower core plate. For creating a downward directed pressure force, the foot must define an annular cavity placed in commmunication by the guide tubes with a zone situated above the core. Such an arrangement has numerous drawbacks: the pressure which prevails in the chamber will depend enormously on the pressure loss undergone by the water which penetrates through the orifices during passage through the guide tubes. This pressure drop, very small when the control bars are out of the core, will become very large when they are in the core. So that the hydraulic holding down force remains sufficient when the bars are inserted, it will be necessary to have a very high water flow rate through the guide tubes when the control bars are out of the core. Now, this very high flow rate short circuits the core, lowers the temperature at the outlet of the core and considerably reduces the thermal efficiency.